The present invention relates to a copper-containing catalyst, a process for the preparation thereof and uses of the same.
Copper-containing catalysts, for example, catalysts containing copper and zinc oxide or catalysts containing copper, zinc oxide and alumina, have been used widely in industries as conventional catalysts in many processes such as low temperature transformation, methanol synthesis, hydro/dehydrogenation processes and so on.
Copper-containing catalysts are generally prepared by a coprecipitation method, that is, by adding a basic coprecipitant, for example, an alkali metal salt such as sodium carbonate, sodium bicarbonate and ammonium carbonate, to a mixed solution of a soluble copper salt, zinc salt and aluminum salt to precipitate out copper, zinc and aluminum as insoluble subcarbonates, which is then filtered, washed, dried, calcined, and pressed and moulded into a catalyst. EP 125,689 discloses a process for preparing a copper-containing catalyst used in methanol synthesis process, wherein the catalyst has an atomic ratio of copper to zinc of 2.8-3.8 (corresponding to 26.9-36.5 parts of zinc oxide per 100 parts of copper oxide), and the parts of alumina is 8-12. In the preparation process, copper and zinc are introduced into the catalyst by a coprecipitation method by adding such a precipitant as sodium carbonate to a solution of the metal salts, and alumina is introduced in the form of aluminum hydroxide sol into the catalyst. U.S. Pat. No. 4,876,402 discloses a process for preparing a catalyst containing copper and zinc oxide for the hydrogenation of aldehydes in gas phase by using sodium carbonate as coprecipitant. In the preparation process of the catalyst the resultant precipitate needs to be pulped, washed and filtered for 4 times in order to remove the sodium salt from the coprecipitate. Nevertherness, as admitted in the prior arts including U.S. Pat. No. 3,303,001 have recognized, copper oxide/zinc oxide catalysts prepared by the standard coprecipitation technique of the prior art will still contain a small amount of sodium. However, the presence of sodium in the catalysts is undesirable because alkali metals, in particular sodium, will diminish the activity of the catalysts. In addition, in the prior arts, copper-containing catalysts are prepared by a coprecipitation method with a basic substance, especially, sodium carbonate, as coprecipitant. Such a preparation process is carried out under a basic condition and zinc compound was precipitates first followed by copper compound, thus it is liable to form inhomogeneous coprecipitates, resulting in non-uniform catalyst crystallite sizes (1.0-10 nm) in irregular crystal shapes, of which the larger crystallites are 10 times the size of the smaller. In order to obtain catalysts of excellent activity and stability, the crystallites of copper oxide should be evenly separated by zinc oxide, but this cannot be achieved by the sodium carbonate method of the prior art. Another disadvantage of using sodium carbonate as coprecipitant is that, since the resulting coprecipitate shall be pulped, washed and filtered repeatedly to remove the undesired sodium salt, it consumes large amounts of pure water and as a result, a large quantity of waste water is discharged and needs to be treated or otherwise will pollute the environment, so the complexity of preparation and the production costs of the catalyst are further increased. The last point, but not least in importance, to be mentioned is that, the specific surface area of the catalysts prepared according to the prior art process is not large enough, and the pore volume and bulk specific weight are all relatively low, so, with respect to the catalytic performance, the catalysts exhibit unsatisfactory activity and selectivity and poor stability.
Therefore, there is a need in the art to develop a copper-containing catalyst having uniform crystallite distribution, large specific surface area and pore volume and, high activity and good stability, and a process for the preparation thereof.
After extensive studies and experiments, the inventors have discovered a novel process comprising a coprecipitation method for the preparation of a copper-containing catalyst featuring a uniform crystallite distribution and showing excellent catalytic performances.
An object of the invention is to provide a copper-containing catalyst featuring a uniform crystallite distribution and showing excellent catalytic performances.
Another object of the invention is to provide a process for preparing a copper-containing catalyst by a coprecipitation method, comprising the step of mixing a working solution containing soluble metal salts for coprecipitation and a solution containing organic acid(s) and/or ammonium salt(s) thereof as coprecipitant to coprecipitate out a mixture of insoluble metal salts containing copper. Said process has overcome the problems of environmental protection existing in the prior art, while the costs for production and starting materials are reduced.
A further object of the invention is to provide a use of the copper-containing catalyst according to the present invention in various chemical processes in which the catalytic action of a copper-containing catalyst is needed, including hydrogenation of aldehydes and/or ketones in gas phase, dehydrogenation of alcohols in gas phase, and a process of methanol synthesis from a mixed gas of CO, CO2 and H2.
These and other objects of the invention will become apparent to the person skilled in the art after reading the specification.
The copper-containing catalyst of the present invention is prepared by a novel process comprising a coprecipitation method and has a uniform crystallite distribution wherein the crystallites having a diameter of less than 1.0 nm account for 0-20%, preferably 0-15%, more preferably 0-10% and most preferably 2-5%; those of 1.0-2.0 nm account for 70-99%, preferably 75-98%, more preferably 80-95% and most preferably 85-90%; and those of more than 2.0 nm account for 0-20%, preferably 0-15%, more preferably 0-10% and most preferably 2-5%.
The copper-containing catalyst of the present invention comprises copper oxide of 30-70 wt %, preferably 33-50 wt %; zinc oxide of 30-70 wt %, preferably 50-65 wt %; and alumina of 0-30 wt %, preferably 10-25 wt % based on the weight of the catalyst.
The copper-containing catalyst of the present invention has a specific surface area of 30-50 m2/g, preferably 35-45 m2/g; a pore volume of 0.10-0.25 ml/g, preferably 0.15-0.20 ml/g; and an average pore diameter of 15-20 nm.
Since sodium salt is not used as a precipitant in the preparation process, sodium can be avoided being introduced into the catalyst. The copper-containing catalyst of the present invention preferably contains no sodium.
The process for preparing the catalyst according to the present invention comprises the steps of:
mixing a working solution containing soluble metal salts for coprecipitation and a solution containing organic acid(s) and/or ammonium salt(s) thereof as coprecipitant to coprecipitate out a mixture of insoluble metal salts; aging and filtering the mixture to obtain a filter cake, drying and calcining to form mixed oxides of catalyst, and then pressing and moulding the mixture to obtain the catalyst which can be in any suitable shape, such as tablet, cylindrical, bar, spherical and the like.
Said soluble metal salts are copper salt(s) and the salt(s) of other metal(s) used as essential metal components of the catalyst, which can be, for example, either copper salt(s), zinc salt(s) and aluminum salt(s), or copper salt(s) and zinc salt(s), selected from the group consisting of chlorides, sulfates, nitrates and acetates. Said organic acid(s) as coprecipitant can be one or more soluble organic acid(s) and may be selected from the group consisting of oxalic acid, malonic acid, succinic acid, and glutaric acid and ammonium salts thereof, preferably malonic acid, oxalic acid and ammonium salts thereof.
The coprecipitation process of the metal salts comprises:
preparing a working salt solution for coprecipitation using the metal salts as starting materials, having a specific concentration of 0.10-0.80M, preferably 0.30-0.50M;
preparing a coprecipitant solution using organic acid(s) and/or ammonium salt(s) thereof as starting material, having a specific concentration of 0.1-0.8M, preferably 0.3-0.5M, and a pH value of 3.0-7.0, preferably 4.0-6.0; wherein the amount of said coprecipitant used in the coprecipitation process exceeds by 5-20 wt %, preferably by 10-15 wt % over the stoichiometrical amount of the metal ions completely precipitated from the working salt solution;
mixing and coprecipitating the above two solutions at a temperature of 15-70xc2x0 C., preferably 25-45xc2x0 C. with stirring and heat preservation to obtain a coprecipitate in suspension; and
aging under a heat preservation condition, filtering and drying in natural air and so on to obtain a coprecipitate filter cake.
The coprecipitation process mentioned above may be conducted by adding the working salt solution to the precipitant solution, or vice versa, or by heating them separately and adding them simultaneously into a precipitation tank.
The precipitation process mentioned above may be carried out by adding the working salt solution and the precipitant solution separately but simultaneously into two parallel elevated tanks, heating respectively to 15-70xc2x0 C., preferably 25-45xc2x0 C., and then adding them simultaneously into a precipitation tank at a lower position with stirring and heat preservation.
The catalyst of the present invention can be prepared by drying and calcining the coprecipitate filter cake obtained above, pressing and moulding the obtained mixture of the oxides optionally together with a given amount of a pressing assistant to obtain the catalyst. The drying is carried out at a temperature of 80-150xc2x0 C., preferably 100-120xc2x0 C., for 6-20 hours, preferably for 8-10 hours; the calcination is carried out at a temperature of 300-500xc2x0 C., preferably 340-370xc2x0 C., for 2-8 hours, preferably for 4-5 hours; the weight ratio of the pressing assistant to dry substrate is 2.0-5.0 wt %, and said pressing assistant may be graphite and/or stearic acid and the like.
The oxides obtained from the steps of drying and calcining and so on mentioned above may be mixed homogeneously with the pressing assistant and a binder, then pressed and moulded to obtain the catalyst. The binder may be zeolite, silicon carbide, silica, silica-alumina, silicates, aluminates and borates and the like.
The catalyst according to the invention needs to be reduced before use just as the ordinary copper-containing catalysts do. The reduction medium may be pure hydrogen gas or hydrogen-containing nitrogen gas. The reduction of the catalyst is carried out by elevating the temperature of the reduction medium to a given level and maintaining the temperature constant for a certain period of time, then the temperature is lowered to the reaction temperature and the feedstock may be fed in to start the catalystic reaction. In order to avoid an undue temperature rise or even a runaway temperature to overburn the catalyst during the reduction process affecting the catalytic performance of the catalyst, the reduction medium is preferably a nitrogen gas containing 5 vol % of hydrogen, and the rate of rise of temperature should be strictly controlled so that the temperature rise in the catalyst bed is less than 20xc2x0 C., while the reduction temperature is at 200-250xc2x0 C., preferably 210-230xc2x0 C.
The catalyst according to the invention is suitable for use in various chemical processes, for example, the process of methanol synthesis from a mixed gas of CO, CO2 and H2, and the low temperature shift process, which need the catalytic action of a copper-containing catalyst, such as a copper/zinc oxide or a copper/zinc oxide/alumina catalyst, and is especially suitable for hydrogenation of aldehydes and/or ketones in gas phase and dehydrogenation of alcohols, for example, the hydrogenation of linear or branched and saturated or unsaturated aldehydes and/or ketones having 2-22 carbon atoms, in particular the mixture of aldehydes derived from an oxo synthesis or a part thereof such as n-butanal, iso-butanal or 2-ethylhexenal, into the corresponding alchols; or the dehydrogenation of alcohols having 2-22 carbon atoms such as iso-propanol or sec-butanol into the corresponding ketones such as acetone or methyl ethyl acetone.
The above chemical processes using the catalyst of the present invention can be carried out in a conventional way of the art, for example, those described in U.S. Pat. Nos. 4,279,781 and 4,876,402 and in Examples 4-6 herein.
Compared with the prior arts, the catalyst according to the present invention and the preparation process thereof do not involve in washing off sodium with pure water, so the preparation process is simplified and rid of the problems of environmental protection and waste water treatment as that arisen in the prior art; particularly, the preparation process of the catalyst according to the present invention has good repeatability since the intermediate insoluble salt coprecipitates of the process are uniform in structure; and moreover since copper and zinc compound are precipitated simultaneously during the coprecipitation process to form crystal precipitate in uniform and superfine particles, the catalyst obtained therefrom has comparatively large specific surface area, pore volume, pore diameter and high bulk specific density, therefore the catalyst of the present invention is superior to that prepared by the conventional process using sodium carbonate in activity, selectivity and stability. Besides, the costs for the production and starting materials of the catalyst according to the invention are reduced.